Manufacturing method of high entropy alloy coating layer

ABSTRACT

A manufacturing method of a high entropy alloy (HEA) coating is provided. The manufacturing method of the HEA coating includes: melting a HEA material, wherein the HEA material comprises at least four metal elements, and the at least four elements are contained in the HEA material with substantially the same content; performing a gas atomization process on the molten HEA material to form HEA powders; and performing a plasma spray process to heat the HEA powders, and spray the heated HEA powders onto a target substrate.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan application serial no. 107140163, filed on Nov. 13, 2018. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.

BACKGROUND Technical Field

The present invention is related to a manufacturing method of a coating layer, and more particularly, to a manufacturing method of a high entropy alloy (HEA) coating layer.

Description of Related Art

High entropy alloy (HEA) is an alloy material that contains four or more major elements. Each major element is contained in HEA with a high atomic percentage, but the atomic percentage of each major element in HEA is not greater than 35 at %. Accordingly, a high entropy effect resulted from the composition of HEA can be effectively generated. As a result of random distribution of different major elements, formation of brittle compound can be avoided. Therefore, HEA may have a high material toughness.

HEA may be applied as a bulk material or as a coating layer. Current manufacturing method of a HEA coating layer includes forming a HEA bulk material by a fusing process, then crushing the HEA bulk material to form HEA powders. Thereafter, the HEA powders are coated on a target substrate. However, defects are likely to form in HEA during the crushing process, and the heat generated by the crushing process may have an influence on material characteristics of HEA. As such, properties of the HEA coating layer are difficult to be controlled.

SUMMARY

Accordingly, the present invention provides a manufacturing method of a HEA coating layer, which is capable of effectively controlling properties of the HEA coating layer.

A manufacturing method of a HEA coating layer in the present invention comprises: melting a HEA material, wherein the HEA material comprises at least four metal elements, and the at least four elements are contained in the HEA material with substantially the same content; performing a gas atomization process on the molten HEA material to form HEA powders; and performing a plasma spray process to heat the HEA powders, and spray the heated HEA powders onto a target substrate.

In some embodiments, the molten HEA material is rapidly cooled down from a high temperature to a low temperature during the gas atomization process, the high temperature ranges from 1150° C. to 1500° C., and the low temperature ranges from 100° C. to 250° C.

In some embodiments, the HEA powders homogeneously consist of a low temperature crystalline phase.

In some embodiments, the low temperature crystalline phase is a body-centered cubic (BCC) phase.

In some embodiments, the HEA powders consist of a low temperature crystalline phase and a high temperature crystalline phase, and a content of the high temperature crystalline phase in the HEA powders is greater than 0%, and less than 10%.

In some embodiments, shapes of the HEA powders are substantially spherical, and an average diameter of the HEA powders ranges from 60 μm to 90 μm.

In some embodiments, a working gas of the plasma spray process comprises an Ar gas and a H₂ gas, and a ratio of a flow rate of the Ar gas with respect to a flow rate of the H₂ gas ranges from 1.5:1 to 34:1.

In some embodiments, a power of the plasma spray process ranges from 20 kW to 55.5 kW.

A HEA coating layer, formed by the above-mentioned method, wherein a content of a high temperature crystalline phase in the HEA coating layer is less than 10%, and greater than or equal to 0%.

In some embodiments, a hardness of the HEA coating layer ranges from 230 HV to 600 HV, and the HEA coating layer is ferromagnetic.

In some embodiments, the high temperature crystalline phase is a face-centered cubic (FCC) phase.

In some embodiments, the HEA coating layer is substantially free of the high temperature crystalline phase.

In some embodiments, the HEA coating layer homogeneously consists of a low temperature crystalline phase.

In some embodiments, the low temperature crystalline phase is a body-centered cubic (BCC) phase.

As above, according to embodiments of the present invention, the HEA powders are formed by a gas atomization process, then the heated HEA powders are heated to a semi-molten state and sprayed to the target substrate by a plasma spray process, so as to form the HEA coating layer. As compared to the HEA powders prepared by crushing a HEA bulk material, the HEA powders obtained from the solidification of the molten HEA material during the gas atomization process may have advantages including having a homogeneous crystalline phase, low impurity content, consistent shape and great flowability and so forth. In addition, during the gas atomization process, the molten HEA material are rapidly cooled down. Therefore, microstructure of the formed HEA powders can be maintained as the low temperature crystalline phase. Furthermore, since the heating on the HEA powders during the plasma spray process is performed in a very short time, the high temperature crystalline phase is not likely to appear in the formed HEA coating layer. Accordingly, the obtained HEA coating layer can homogeneously consist of the low temperature crystalline phase. In some embodiments, the HEA coating layer may be further ensured to homogeneously consist of the low temperature crystalline phase by sieving the HEA powders, controlling power and gas flow rate for generating the plasma, and cooling the target substrate. Thereby, the manufacturing method of the HEA coating layer of the present invention can effectively control the crystalline phase of the HEA coating layer. In other words, various physical properties of the HEA coating layer can be more precisely adjusted.

To make the aforementioned more comprehensible, several embodiments accompanied with drawings are described in detail as follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of the disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the disclosure and, together with the description, serve to explain the principles of the disclosure.

FIG. 1 is a schematic diagram illustrating a HEA coating layer manufacturing apparatus according to some embodiments of the present invention.

FIG. 2A is a scanning electron microscope (SEM) image of the HEA powders used for experimental examples 1-4.

FIG. 2B is a SEM image of the HEA powders used for experimental examples 6 and 7.

FIG. 3 are X-Ray diffraction (XRD) spectrums of the HEA powders used for experimental examples 1 through 7.

FIG. 4 is a diagram illustrating magnetization value of the HEA powders used for experimental examples 1-7 with respect to magnetic field.

FIG. 5 are XRD spectrums of the HEA coating layer of experimental examples 1-7.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1 is a schematic diagram illustrating a HEA coating layer manufacturing apparatus 10 according to some embodiments of the present invention. In some embodiments, a HEA coating layer CL may be formed on a target substrate TS by using the HEA coating layer manufacturing apparatus 10.

Referring to FIG. 1, the HEA coating layer manufacturing apparatus 10 may include a gas atomization apparatus 100. In some embodiments, the gas atomization apparatus 100 includes a melting part MP. A solid HEA material is fused in the melting part MP to form a molten HEA material. In some embodiments, a crucible C is disposed in the melting part MP. The solid HEA material may be placed in the crucible C, and is heated to form the molten HEA material by, for example, a manner of induction heating. At this time, a temperature of the molten HEA material may range from 1150° C. to 1500° C. In some embodiments, the melting part MP may be kept in a vacuum state while heating the HEA material. In addition, inert gas and/or reducing gas may be introduced into the melting part MP, so as to avoid from oxidization of the HEA material. For instance, the inert gas may include He gas, Ar gas or the like, whereas the reducing gas may include N₂ gas, H₂ gas or the like. In some embodiments, the HEA material includes four or more metal elements. These metal elements are all major elements of the HEA material, and are contained in the HEA material with about the same atomic percentage. For instance, an atomic percentage of each metal element in the HEA material ranges from 5 at % to 35 at %. In some embodiments, the metal elements of the HEA material may be at least four metal elements selected from a group consisting of Ni, Fe, Co, Cr, Al, Ti, Zr, Cu, Mn and Si. For instance, the HEA material may be Ni₂FeCoCrAl_(x)Ti_(y), of which x ranges from 0 to 1.5, and y ranges from 0 to 1.0.

In some embodiments, the gas atomization apparatus 100 further includes an atomization part AP. The atomization part AP may be communicated with the melting part MP. As such, the molten HEA material can enter the atomization part AP from the melting part MP. In some embodiments, the atomization part AP includes a nozzle NZ. The molten HEA material may enter the nozzle NZ from the melting part MP, and may be ejected by the nozzle NZ as a fluid. In addition, the atomization part AP may further include a gas source GS. The gas source GS may be disposed at a chamber wall of the atomization part AP, and is configured to generate a high pressure gas flow (indicated as the arrows close to the nozzle NZ in FIG. 1) in the atomization part AP. This high pressure gas flow may be guided to hit the HEA fluid ejected from the nozzle NZ, so as to form small droplets. These droplets fly in the atomization part AP, and are solidified to form HEA powders. In some embodiments, the gas from the gas source GS may include inert gas and/or reducing gas. For instance, the inert gas may include He gas, Ar gas, N₂ gas etc., whereas the reducing gas may include H₂ gas or the like. By introducing the inert gas and/or the reducing gas into the atomization part AP, oxidization of the HEA fluid during solidification may be further avoided. Moreover, in some embodiments, the atomization part AP may further include a gas pumping system (not shown), which is configured to keep the atomization part AP in a vacuum environment.

In some embodiments, the atomization part AP may further include powder collection boxes CB. The powder collection boxes CB may be connected to the chamber wall of the atomization part AP, and are configured to collect the HEA powders. In some embodiments, the atomization part AP may include multiple powder collection boxes CB, and the powder collection boxes CB may be disposed at different locations on the chamber wall of the atomization part AP. In general, the closer the powder collection box CB is to the nozzle NZ, the smaller the HEA powders can be collected. On the other hand, the more distant the powder collection box is from the nozzle NZ, the larger the HEA powders can be collected. Taking the gas atomization apparatus 100 as an example, the atomization part AP may include two powder collection boxes CB, which are respectively disposed at a sidewall and a bottom portion of the atomization part AP. However, those skilled in the art may adjust the amount and locations of the powder collection boxes CB according to process requirements, the present invention is not limited thereto. In addition, in some embodiments, the powder collection boxes CB disposed at the chamber wall of the atomization part AP may be communicated with the gas source GS, so that the gas flowing into the atomization part AP may return to the gas source GS. The HEA powders collected via this gas atomization process can have consistent shape and great flowability. In some embodiments, the HEA fluid may be rapidly solidified from a high temperature of about 1500° C. to a low temperature no greater than about 300° C. (e.g., from 100° C. to 250° C., or from 150° C. to 250° C.), so as to form the HEA powders. Furthermore, the solidification time may be less than or equal to 5 seconds. In some embodiments, due to the rapid solidification, substantially the whole HEA powders can be maintained as a low temperature crystalline phase. In other words, substantially the whole HEA powders consist of the low temperature crystalline phase. In alternative embodiments, the obtained HEA powders consist of the low temperature crystalline phase and trace of a high temperature crystalline phase. For instance, a percentage of the high temperature crystalline phase in the HEA powders may be less than 10%, such as in a range from 0% to 5%, or from 0% to 3%. In some embodiments, the HEA material is NiFeCoCrAl. In these embodiments, the low temperature crystalline phase is a body-centered cubic (BCC) phase, whereas the high temperature crystalline phase is a face-centered cubic (FCC) phase.

The HEA coating layer manufacturing apparatus 10 may further include a plasma spray apparatus 110. The HEA powders obtained via the gas atomization apparatus 100 are fed into the plasma spray apparatus 110. The plasma spray apparatus 110 is configured to heat the HEA powders, and spray the soften HEA powders to the target substrate TS. Accordingly, the HEA coating layer CL can be formed on the target substrate TS.

In some embodiments, the plasma spray apparatus 110 includes an electrode E1 and an electrode E2, which are configured to generate plasma. The electrode E1 may be functioned as an anode. In some embodiments, the electrode E1 has a nozzle portion. In these embodiments, the nozzle portion of the electrode E1 has an opening P. In addition, the electrode E2 is disposed aside the electrode E1, and may be functioned as a cathode. Working gas flows to the region between the electrode E1 and the electrode E2 via a gas inlet IN. In some embodiments, the working gas includes Ar gas and diatomic gas. For instance, the diatomic gas may be H₂ gas. The working gas in between the electrode E1 and the electrode E2 may be ionized by a bias voltage between the electrode E1 and the electrode E2, so as to form the plasma. The plasma may be ejected from the opening P of the electrode E1, so as to form a plasma jet J. In some embodiments, a HEA powder intake F may be located in the vicinity of the opening P of the electrode E1, such that the HEA powders fed into the plasma spray apparatus 110 can be immediately heated. In some embodiments, the HEA powders may be heated to a semi-molten state, and sprayed to a surface of the target substrate TS along a flow direction of the plasma jet. The soften HEA powders may stack on the target substrate TS to form the HEA coating layer CL. In some embodiments, the plasma spray apparatus 110 may further include a cooling system CS. The cooling system CS may be in thermal contact with the electrode E1 and the electrode E2, so as to dissipate heat from the electrode E1 and the electrode E2. In some embodiments, the cooling system CS may be a water cooling system CS, but the present invention is not limited thereto.

Since the HEA powders are heated by exposing to the plasma in a very short time (e.g., less than 10 μs), the obtained HEA powders can have a homogeneous crystalline phase. In some embodiments, the HEA powders may be sieved before being fed to the plasma spray apparatus 110. An average diameter (also referred as “D50”) of the sieved HEA powders may range from 60 μm to 90 μm. In these embodiments, the HEA powders are not likely to be heated thoroughly, so as to maintain in the low temperature crystalline phase, and have a great powder flowability. Therefore, in some embodiments, substantially the whole HEA coating layer CL consists of the low temperature crystalline phase. In alternative embodiments, the HEA coating layer CL mainly includes the low temperature crystalline phase, and also includes trace of the high temperature crystalline phase. For instance, a percentage of the high temperature crystalline phase in the HEA coating layer CL may be less than 10%, such as in a range from 0% to 5%, or from 0% to 3%. On the other hand, if the HEA powders fed into the plasma spray apparatus 110 have an average diameter less than 60 μm, the high temperature crystalline phase may appear due to thorough heating by the plasma. In addition, if the HEA powders fed into the plasma spray apparatus 110 have an average diameter greater than 90 μm, feeding of the HEA powders may be hindered because of a low flowability.

In some embodiments, the HEA material is NiFeCoCrAl. In these embodiments, the low temperature crystalline phase is the BCC phase, whereas the high temperature crystalline phase is the FCC phase. As compared to the FCC phase, the BCC phase has fewer dislocation systems, so as to have a greater mechanical strength (e.g., hardness). In addition, the BCC phase is ferromagnetic, and has a rather high saturation magnetization value. On the other hand, the FCC phase is paramagnetic, and has a rather low saturation magnetization value. In some embodiments, substantially the whole HEA coating layer CL consists of the BCC phase, and the HEA coating layer CL may be served as a functional coating layer that enhance hardness of a workpiece, or served as a ferromagnetic coating layer. In these embodiments, the hardness of the HEA coating layer CL ranges from 230 HV to 600 HV, or from 238 HV to 427 HV. In addition, the saturation magnetization value of the HEA coating layer CL may range from 10 emu/g to 100 emu/g.

In addition to controlling the diameter of the HEA powders, parameters of power and gas flow rate for generating the plasma can be adjusted to further control the heating of the HEA powders. The higher the temperature of the plasma, the more likely the HEA powders are thoroughly heated, and the more likely the high temperature crystalline phase appears. Therefore, microstructure of the HEA coating layer CL may be further controlled to be the low temperature crystalline phase by adjusting the power and the gas flow rate for generating the plasma. In some embodiments, the power for generating the plasma may range from 20 kW to 55 kW. In addition, the temperature of the plasma may be raised by increasing the flow rate of the Ar gas, whereas the plasma may be stably maintained by increasing the flow rate of the diatomic gas. In some embodiments, a ratio of the flow rate of the Ar gas with respect to the flow rate of the diatomic gas may range from 1.5:1 to 34:1. In these embodiments, the flow rate of the Ar gas may range from 40 slpm to 75 slpm, whereas the flow rate of the diatomic gas may range from 1.5 slpm to 9.5 slpm. Moreover, in some embodiments, the target substrate TS may be cooled during the spray process. As such, the microstructure of the HEA coating layer can be further ensured to be the low temperature crystalline phase.

As above, according to embodiments of the present invention, the HEA powders are formed by a gas atomization process, then the heated HEA powders are heated to a semi-molten state and sprayed to the target substrate by a plasma spray process, so as to form the HEA coating layer. As compared to the HEA powders prepared by crushing a HEA bulk material, the HEA powders obtained from the solidification of the molten HEA material during the gas atomization process may have advantages including having a homogeneous crystalline phase, low impurity content, consistent shape and great flowability and so forth. In addition, during the gas atomization process, the molten HEA material are rapidly cooled down. Therefore, microstructure of the formed HEA powders can be maintained as the low temperature crystalline phase. Furthermore, since the heating on the HEA powders during the plasma spray process is performed in a very short time, the high temperature crystalline phase is not likely to appear in the formed HEA coating layer. Accordingly, the obtained HEA coating layer can homogeneously consist of the low temperature crystalline phase. In some embodiments, the HEA coating layer may be further ensured to homogeneously consist of the low temperature crystalline phase by sieving the HEA powders, controlling power and gas flow rate for generating the plasma, and cooling the target substrate. Thereby, the manufacturing method of the HEA coating layer of the present invention can effectively control the crystalline phase of the HEA coating layer. In other words, various physical properties of the HEA coating layer can be more precisely adjusted.

Substantial effects of the embodiments in the present invention will be describe with reference to experimental examples 1-7.

[Preparation of the HEA Powders]

Referring to FIG. 1, metal slugs (provided by Alfa Aesar) of Al, Co, Cr, Fe and Ni are melted and evenly mixed in the melting part MP of the gas atomization apparatus 100 to form a molten HEA material. The melting part MP is kept in an Ar atmosphere, and the molten HEA material is stirred by a vibration manner during melting and mixing. The molten HEA material then enters the atomization part AP, and is ejected as fluid from the nozzle NZ. The HEA fluid is hit by a high pressure gas flow to form small droplets. These droplets fly in the atomization part AP, and are rapidly solidified to form HEA powders. Afterwards, the HEA powders are sieved, and some of the HEA powders having an average diameter in a certain range are collected.

[Plasma Spray Process]

Thereafter, the collected HEA powders enter the plasma spray apparatus 110 along with N₂ carrier gas. A feeding rate of the HEA powders is about 30 g/min. In addition, the target substrate TS is provided. The target substrate TS is a 304 stainless steel plate, and are blasted with alumina sand and cleaned prior to the plasma spray process. A plasma generation device of the plasma spray apparatus 110 that includes the electrode E1 and the electrode E2 are mounted on a robotic arm (not shown), so as to control a spray distance and a spray angle during the plasma spray process. The target substrate TS may be moved during the plasma spray process, such that the target substrate TS can be sprayed by a scanning manner. Specifically, the target substrate TS may be moved along a row direction during each of the scanning cycles, and may be moved along a column direction after one scanning cycle (by about 140 mm) for the next scanning cycle. The working gas for generating the plasma includes Ar gas and H₂ gas. The target substrate TS is cooled by an air jet during the plasma spray process, and some of the HEA powders failing to successfully attach to the target substrate TS and/or not getting sufficient heating are removed by the air jet. A thickness of the HEA coating layer CL formed on the target substrate TS is measured by the repeat time of the plasma spray process. In the experimental examples describe below, the plasma spray process is repeated 10 times, and the thickness of the formed HEA coating layer CL ranges from about 150 μm to about 200 μm.

Table 1 lists average diameters of the HEA powders and various parameters for the plasma spray process according to experimental examples 1-7.

TABLE 1 average diameter of power for flow flow the HEA generating rate of rate of powders the plasma Ar gas H₂ gas (μm) (kW) (l/min) (l/min) experimental 10-60 35.3 35 9.3 example 1 experimental 10-60 35.3 50 9.3 example 2 experimental 10-60 42.3 35 9.3 example 3 experimental 10-60 42.3 50 9.3 example 4 experimental 10-60, 60-90 45.8 50 9.3 example5 experimental 60-90 35.3 50 9.3 example6 experimental 60-90 52.9 50 9.3 example7

[Characterization of the HEA Powders]

Appearance and composition of the HEA powders used for experimental examples 1-7 are observed by using a scanning electron microscope (SEM) (JSN6500F, provided by JEOL) equipped with an energy dispersive spectrometer (EDS). In addition, crystalline phase of the HEA powders used for experimental examples 1-7 are analyzed by using an X-Ray diffractometer (XDS2000, provided by Scintag). Specifically, Cu Kα radiation is applied in the X-Ray diffraction (XRD) analysis, and the XRD analysis is performed at room temperature. Furthermore, magnetism of the HEA powders used for experimental examples 1-7 are analyzed by using a magnetic property measurement system (MPMS, provided by Quantum Design) at a temperature ranging from 10 K to 400 K and an applied magnetic field of 0 T to 7 T.

FIG. 2A is a SEM image of the HEA powders used for experimental examples 1-4. FIG. 2B is a SEM image of the HEA powders used for experimental examples 6 and 7.

Referring to FIG. 2A and FIG. 2B, the HEA powders used for experimental examples 1-4, 6 and 7 have a consistent appearance, which is substantially spherical. As shown in FIG. 2B, the HEA powders being sieved to have an average diameter ranging from 60 μm to 90 μm (i.e., the HEA powders for experimental examples 6 and 7) are substantially spherical, and having a greater diameter consistency than the HEA powders shown in FIG. 2A (i.e., the HEA powders for experimental examples 1-4). Although the image of the HEA powders used for experimental example 5 is not provided, the HEA powders used from experimental example 5 can be regarded as a mixture of the HEA powders for experimental examples 1-4 and the HEA powders for experimental examples 6 and 7. As such, the HEA powders used for experimental example 5 may have consistent appearance (e.g., spherical appearance) as well. In addition, table 2 lists composition of the HEA powders for experimental examples 6 and 7, which is measured by the EDS.

TABLE 2 element atomic percentage (at %) Al 20.92 Cr 20.40 Fe 19.47 Co 20.13 Ni 19.08

Referring to table 2, various elements are contained in the HEA powders with about the same content, and may all serve as major elements in the HEA material.

FIG. 3 is an XRD spectrum of the HEA powders used for experimental examples 1-7.

Referring to FIG. 3, data lines DA to DG are XRD spectrums of the unsieved HEA powders. The data line DA indicates the HEA powders that have not been annealed. The data lines DB to DG respectively indicate the HEA powders that have been annealed at 500° C., 600° C., 700° C., 800° C., 900° C. and 1000° C. for 48 hours. A characteristic peak CP1 indicates the BCC phase, whereas a characteristic peak CP2 indicates the FCC phase. Referring to data lines DA to DG, the FCC phase appears when annealing temperature exceeds 600° C., and intensity of the characteristic peak of the FCC phase (i.e., the characteristic peak CP2) increases along with annealing temperature. On the other hand, the HEA powders that have not been annealed and the HEA powders that have been annealed at 500° C. appear to homogenously consist of the BCC phase. In other words, the as-obtained HEA powders from the gas atomization process can homogeneously consist of the BCC phase. It should be noted that, in experimental examples 1-7, the HEA powders fed into the plasma spray apparatus do not subject to an annealing treatment, and homogeneously consist of the BCC phase, as implied by the data line DA.

FIG. 4 is a diagram illustrating magnetization value of the HEA powders used for experimental examples 1-7 with respect to magnetic field.

Referring to FIG. 4, data lines DH to DK respectively indicate a relationship of magnetization value of the unsieved HEA powders with respect to magnetic field. The data line DH indicates the HEA powders that have not been annealed. The data lines DI to DK respectively indicate the HEA powders that have been annealed at 500° C., 1000° C. and 1200° C. Referring to the data lines DH to DK, as annealing temperature increases, saturation magnetization values initially increases, then gradually decreases. Specifically, when annealing temperature is lower than 500° C., saturation magnetization increases along with annealing temperature. When annealing temperature is higher than 500° C., saturation magnetization decrease as annealing temperature increases. Accordingly, the as-obtained HEA powders and the HEA powders that have been annealed at a temperature lower than 500° C. appear to be ferromagnetic, whereas the HEA powders that have been annealed at a temperature higher than 500° C. appear to be paramagnetic. As combining results shown in FIG. 3 and FIG. 4, a transition between ferromagnetism and paramagnetism may be resulted from a transition between the BCC phase and the FCC phase. In other words, the BCC phase is corresponded to ferromagnetism, whereas the FCC phase is corresponded to paramagnetism.

[Analysis on HEA Coating Layer]

The crystalline phase of the HEA coating layer CL is analyzed by an X-Ray diffractometer (XDS2000, provided by Scintag). Specifically, Cu Kα radiation is applied for the XRD analysis, and the XRD analysis is performed at room temperature. In addition, a hardness measurement is performed on the HEA coating layer CL by a Vickers hardness test with loading of 300 g.

FIG. 5 are XRD spectrums of the HEA coating layer of experimental examples 1 to 7.

Referring to FIG. 5, data lines DL to DR respectively indicates an XRD spectrum of the HEA powders of experimental examples 1 to 7. The HEA powders for experimental examples 1 to 7 do not subject to an annealing treatment, and the BCC phase and ferromagnetism are maintained (as shown in FIG. 3 and FIG. 4). The data lines DG to DP implies that the HEA coating layers formed by the HEA powders with average diameter ranging from 10 μm to 60 μm or 10 μm to 90 μm includes a mixture of the FCC phase and the BCC phase (as shown by the appearance of the characteristic peak CP1 and the characteristic peak CP2). Moreover, content of the FCC phase (i.e., intensity of the characteristic peak CP2) increases along with the flow rate of Ar gas. In addition, the data lines DQ and DR implies that the HEA coating layers formed by the HEA powders with average diameter of 60 μm to 90 μm maintain in the BCC phase (i.e., only the characteristic peak CP1 is observed). Even though higher power and larger flow rate of Ar gas are applied in experimental examples 6 and 7 (indicated by the data lines DQ and DR), the FCC phase (i.e., the characteristic peak CP2) does not appear. Therefore, diameter of the HEA powders fed into the plasma spray apparatus is a key factor deciding if the HEA coating layer could be maintained as the BCC phase.

Table 3 lists hardness of the HEA coating layers of experimental examples 1 to 7.

TABLE 3 average hardness (HV) experimental example 1 235.5 ± 30.6 experimental example 2 238.5 ± 13.8 experimental example 3  246 ± 19.6 experimental example 4  249 ± 12.8 experimental example 5 311.1 ± 15.5 experimental example 6 418.7 ± 20.1 experimental example 7 427.9 ± 20.7

Referring to the table 3, as compared to experimental examples 1-5, the HEA coating layers of experimental examples 6 and 7 appear to have greater hardness values. As combining results shown in FIG. 5 and table 3, the HEA coating layers that homogeneously consist of the BCC phase has greater hardness than the HEA coating layers with a mixture of the BCC phase and the FCC phase.

In summary of the above results, the crystalline phase of the HEA coating layer can be controlled as a single phase if the diameter of the HEA powders is within a certain range, as well as properly tuning of the plasma spray process. Therefore, physical properties (e.g., hardness and magnetism etc.) of the obtained HEA coating layer can be successfully controlled.

As above, according to embodiments of the present invention, the HEA powders are formed by a gas atomization process, then the heated HEA powders are heated to a semi-molten state and sprayed to the target substrate by a plasma spray process, so as to form the HEA coating layer. As compared to the HEA powders prepared by crushing a HEA bulk material, the HEA powders obtained from the solidification of the molten HEA material during the gas atomization process may have advantages including having a homogeneous crystalline phase, low impurity content, consistent shape and great flowability and so forth. In addition, during the gas atomization process, the molten HEA material are rapidly cooled down. Therefore, microstructure of the formed HEA powders can be maintained as the low temperature crystalline phase. Furthermore, since the heating on the HEA powders during the plasma spray process is performed in a very short time, the high temperature crystalline phase is not likely to appear in the formed HEA coating layer. Accordingly, the obtained HEA coating layer can homogeneously consist of the low temperature crystalline phase. In some embodiments, the HEA coating layer may be further ensured to homogeneously consist of the low temperature crystalline phase by sieving the HEA powders, controlling power and gas flow rate for generating the plasma, and cooling the target substrate. Thereby, the manufacturing method of the HEA coating layer of the present invention can effectively control the crystalline phase of the HEA coating layer. In other words, various physical properties of the HEA coating layer can be more precisely adjusted.

It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed embodiments without departing from the scope or spirit of the disclosure. In view of the foregoing, it is intended that the disclosure covers modifications and variations provided that they fall within the scope of the following claims and their equivalents. 

1. A manufacturing method of a high entropy alloy (HEA) coating layer, comprising: melting a HEA material, wherein the HEA material comprises at least four metal elements, and the at least four elements are contained in the HEA material with substantially the same content; performing a gas atomization process on the molten HEA material to form HEA powders; and performing a plasma spray process to heat the HEA powders, and spray the heated HEA powders onto a target substrate.
 2. The manufacturing method of the HEA coating layer of claim 1, wherein the molten HEA material is rapidly cooled down from a high temperature to a low temperature during the gas atomization process, the high temperature ranges from 1150° C. to 1500° C., and the low temperature ranges from 100° C. to 250° C.
 3. The manufacturing method of the HEA coating layer of claim 1, wherein the HEA powders homogeneously consist of a low temperature crystalline phase.
 4. The manufacturing method of the HEA coating layer of claim 3, wherein the low temperature crystalline phase is a body-centered cubic (BCC) phase.
 5. The manufacturing method of the HEA coating layer of claim 1, wherein the HEA powders consist of a low temperature crystalline phase and a high temperature crystalline phase, and a content of the a high temperature crystalline phase in the HEA powders is greater than 0%, and less than 10%.
 6. The manufacturing method of the HEA coating layer of claim 1, wherein shapes of the HEA powders are substantially spherical, and an average diameter of the HEA powders ranges from 60 μm to 90 μm.
 7. The manufacturing method of the HEA coating layer of claim 1, wherein a working gas of the plasma spray process comprises an Ar gas and a H₂ gas, and a ratio of a flow rate of the Ar gas with respect to a flow rate of the H₂ gas ranges from 1.5:1 to 34:1.
 8. The manufacturing method of the HEA coating layer of claim 1, wherein a power of the plasma spray process ranges from 20 kW to 55.5 kW.
 9. A HEA coating layer, formed by the manufacturing method of the HEA coating layer of claim 1, wherein a content of a high temperature crystalline phase in the HEA coating layer is less than 10%, and greater than or equal to 0%.
 10. The HEA coating layer of claim 9, wherein a hardness of the HEA coating layer ranges from 230 HV to 600 HV, and the HEA coating layer is ferromagnetic.
 11. The HEA coating layer of claim 9, wherein the high temperature crystalline phase is a face-centered cubic (FCC) phase.
 12. The HEA coating layer of claim 9, wherein the HEA coating layer is substantially free of the high temperature crystalline phase.
 13. The HEA coating layer of claim 9, wherein the HEA coating layer homogeneously consists of a low temperature crystalline phase.
 14. The HEA coating layer of claim 13, wherein the low temperature crystalline phase is a body-centered cubic (BCC) phase. 